Field of the Invention
The present invention describes methods for preparing a pigment particle dispersion utilizing a solid milling agent that is solubilized in the dispersion after a predetermined particle size is obtained. The present invention also describes methods for preparing a pigment particle dispersion eliminating the need for separating the solid milling agent from the dispersion after a predetermined particle size is obtained.
Description of the Related Art
The process of milling has been known and used for hundreds of years in a variety of applications including but not limited to printing arts, pharmaceuticals and foods. Milling typically involves repeated, random collisions of solid particles suspended in a slurry, or liquid dispersion, with milling media by way of impact, shear and cavitation forces over a predetermined period of time. By so doing, the particles break or de-aggregate into small, uniform particle sizes and/or particle size distributions.
Selecting an appropriate milling media type and shape depends upon the application and the desired particle size and/or particle size distribution of the dispersion. Typical milling media includes beads of ceramics, glasses, stainless steel, and solid polymeric resins. Polymeric media can be used to reduce or prevent contamination which may exist when using other types of milling media such as ceramic or metallic media.
The efficiency of reducing particle size and/or obtaining a predetermined particle size distribution of a dispersion also depends upon the size of the milling media. Smaller milling media ranging from about 0.2 mm to 0.65 mm can be used for more demanding applications requiring small particle sizes and higher quality dispersions such as in ink jet inks or high gloss packaging inks. Moreover, milling media less than about 0.1 mm can help reduce the mean particle size of a dispersion to less than 100 nm for a broad range of products, including but not limited to carbon black and organic pigments.
Conventionally, a re-circulating mill, such as those from Eiger, Drais or Netzsch, or discrete pass mills, such as those from Buhler, Premier or Dyno are, are used in the industry to perform milling. Once the predetermined particle size of the dispersion is obtained, milling is stopped. Generally, screens or gaps are employed to retain the milling media in the mill. Another approach is to mix milling media with a dispersion in a mixing tank and filter or separate the media from the dispersion after a predetermined particle size is obtained. The extra time required for filtration or separation reduces throughput. In addition, since all of the milling media cannot be separated from the dispersion during filtration or separation, the final product inevitably includes contaminants. This is especially relevant when ceramics or metals are employed as the milling media.
A need therefore exists in the art for an efficient milling process for producing a pigment particle dispersion of a predetermined particle size. Another needed exists for a less expensive process for producing a pigment particle dispersion of a predetermined particle size. Further, a need exists in the art for a pigment particle dispersion of a predetermined particle size that is substantially free of contamination.